Rotary field chill-mold



uly 12, 19 I o. SCHAABER 2,944,309

ROTARY FIELD CHILL-MOLD Filed Sept. 2. 195::

INVENTOR 0:10 Scbaaber;

BY Wwvwgafl ATTORIHEYS 2,944,309 ROTARY FIELD CHILL-MOLD Otto Schaaber,Bremen-St. Magnus, Germany (Brauteichen 23, Bremen-Schonebeck, Germany)Filed Sept. 2, 1954, Ser. No. 453,967

Claims priority, application Germany Sept. 4, 1953 8 Claims. (Cl. 22-573) It was hitherto customary when employing'magnetic rotary fields 'inconnection with chill-molds for metal casting either to provide a layerof'insulating' material between the iron poles serving for producing therotary field or to separate the field producing parts from the liquidmetal by a wall of non-magnetic steel and possibly by a brick lining.

Such measures are unsuitable when using magnetic rotary fields inwater-cooled chill-molds and especially in the case of tubular moldssuch as are used in continuous casting, becausethe increase in outputwhich it is endeavoured to obtain can only be achieved with thesechill-molds if good heat conducting is insuredbetween the molten metaland the cooling liquid. The use of magnetic rotary fields in continuouscasting is similar to that described in my co-pending application SerialNo. 376,917, filed August 27, 1953 for Casting Process, now Patent2,877,525, issued March 17, 1959 and in the Junghans and Schaaberapplication filed Sept. 4, 1951, Serial No. 245,014 for Method forCasting Metals, now abandoned. The shaping or shape-giving portion ofthe mold must therefore be of a material having good heat conductingproperties, such as are generally possessed by metals. Consequently alarge percentage of the continuous casting chill-molds in practical useare made of pure copper.

From the point of view of the lowest possible resistance to the passageof heat it is endeavoured to make the walls as thin as possible;practical requirements, however, oppose any reduction in wall thicknessbecause the chill-molds must not become distorted under relatively greatdifferences in temperature and must therefore be sufiiciently stable.Furthermore, continuous castmg molds are subjected to considerablemechanical stressing when introducing and removing the control pin.Finally, the chill-molds must have a sufiiciently great wall thicknessto compensate for wear of the shapegiving surfaces by reconditioning, onwhichaccount the walls must not be too thin.

This invention is described with reference to the accompanying drawingsin which:

Figure 1 is a longitudinal sectional view through a continuous castingmold having a rotating magnetic field applied thereto; and

Figure 2 is a cross-sectional view taken on the line 2--2 of Figure 1.

Molten metal is poured from ladle 2 intorunnerl 4 from which it flowsinto the continuous casting chillmold 6 having a water-cooled jacketsurrounding at least the part of the mold body giving shape to the ingotbeing formed in the mold. The metal solidifies in the mold to form an atleast partially solid ingot 8 which is withdrawn from the lower open endof the mold.

Surrounding the body of mold 6 are a plurality of roice tary field poles10a, 10b and 10c, each having its respective coil 12a, 12b and 120, thecoils in turn being connected through transformer T and switchS toalternating current electric power line -P. Upon the closing of switch8, coils 12a, 12b and 120 are successively energized and thereby induceda rotating magnetic field within the interior of mold 6. The heatconductivity and magnetic shielding characteristics of the materialforming the body of mold 6 thus are of considerable importance.

Tests carried out with tubular chill-molds of electrolytic copper, whichmaterialwas chosen on account of its high heat conductivity, showed thatit was not possible to obtain a rotary field of suitable strength in theinterior of the mold. Obviously this is due to the screening of themagnetic field as a result of induced eddy currents. Measurements takenwith the aid of a rotary field measuringinstrument (see Kohlrausch, 1951edition, volume 2, page 163) gave the results of the screening effect ofa copper tube with an internal diameter of 112 mms. indicated in Table1, column 2.

Table I VROIAIION MOMENT IN THE TUBE Rotation moment undisturbed Wallthickness, mms.

Cu-tube MS 63-tube 0.01 0.30 0. 02 0. as 0.027 0. 40 0. O4 0. 436 0. 060. 516 0.11 0.60 0.18 0. 634 0.23 0.70 0.28 0. 714

The table shows that even with a wall thickness of only 3 mms. therotation moment forming in the interior of the copper tube (designatedas Rotation moment in the tube in the table)--for the production ofwhich alternating current of Hertz was used-amounted to only about 28%of the rotation moment produced without using the copper cylinder (inthetable designated as Rotation moment undisturbed). The screening effectof the copper tube therefore amounted to 72%. Moreover a wall thicknessof 3 mms. is undesirably low for practical foundry work.

In order to obtain less screening of the rotary field within the moldWall, the problem arising is to use metals having a lower electricalconductivity without the heat conductivity dropping at the same time toosteeply.

According to the Wiedemann and Frantz law a certain constantrelationexists theoretically for all scales between the heatconductivity A and the specific electric resistance p at a givenabsolute temperature T. Actually, however, the relation between heatconductivity and electrical conductivity deviates not inconsiderably forsome scales; see Table II in which for different metals the heatconductivity 7\ is given in cal./-C.cm.sec., specific electricresistance p in ,utl-cm. and theprodu ct of both divided by absolutetemperature T=293K. Theoreti cally Volt 2 For rotary field chill-molds,especially those for continuous casting, materials appear appropriatewhich, with a heat conductivity which is lower than that of copper butpreferably not below of that of copper, have a specific electricresistance so much higher than copper that the product of heatconductivity and. specific resistance is higher than that for copperalone.

Measurements taken on continuous casting chill-molds with the rotaryfield measuringinstrument showed the values also set forth in Table Ifor the screening of the rotary field in a tube made of brass alloy MS63 having the same internal diameter and the same length as the Cu-tubementioned above. It will be recognized that with a wall thickness of 6mms. a screening as regards the rotation moment obtained is less than50%, and in this connection it should be mentioned that this wallthickness is entirely satisfactory from the point of view of foundrywork. Table II also shows that of the materials mentioned therein, inaddition to brass and beryllium-copper, chrome, tungsten and siliconappear very suitable. in Table II brass with 63% copper and 37% zinccontent is designated by MS 63. The values given for berylliumcopperapply for the copper alloy AT hardened by precipitation hardening andquenching and with a content of 2% beryllium and 0.25% cobalt.

Particularly in the case of semi-conductors, of which silicon is atypical example, it is found that a relatively high heat conductingcapacity is combined with a very low electricity conducting capacity.

In the same way as semi-conductors, certain metal oxides, sinteredoxides and sintered metals also appear very promising.

Beryllium copper, preferably the above mentioned beryllium copper with acontent of 2% beryllium and 0.2% cobalt, would seem to be particularlysuitable for the above-mentioned purpose.

The physical data for beryllium copper are more favorable for theconstruction of rotary field chill-molds than the physical data whichcome into question of electrolytic copper and even of brass, whichlatter is already more advantageous than copper.

The heat conductivity of beryllium copper is 0.25 cal./cm.C. sec., thatis about 26% that of copper. The electric conductivity of berylliumcopper amounts to about 17 to 20% of that of copper. This means afavorable relationship betwcen electric and thermic conductivity, evenmore favorable than, for example, in the case of brass. In accordancetherewith, the Wiedmann-Frantz constant for beryllium copper lies about1.3 to 1.5 times higher than for copper.

In addition, there is the fact that the tensile strength of hardenedberyllium copper is much higher than that of copper. Whereas the tensilestrength of hardened beryllium copper is about 130 kgs./mm. the tensilestrength of electrolytic copper, if it is annealed, is 20 'kgs/mmfi; ifcold rolled copper, has a tensile strength of 35 kgs./mm.

Compared with a chill-mold made of copper plates in semi-hardenedcondition, for which a tensile strength of 30 kgs./mm. can be assumed,the same rigidity is therefore to be expected if, in the case of aberyllium copper mold, a wall thickness of only 23% of that of copper isused.

It must also be taken into consideration that the elastic limit ofhardened out beryllium copper amounts to about 98 kgs./rnm. whereas inthe case of electrolytic copper it is reachedat a tension of only 15kgs./mrn. In the case of brass the tensile strength is about 50kgs./rnm. and the elastic limit about 35 kgs./mm. The elastic limit ofthe material is the determining factor as to whether under constantheating. the reversibility of the expansion process would be exceeded. Abody becomes distorted when during the process of heating and recoolingthe elastic limit is exceeded as a result of the body being unequallyheated. In this respect a high elastic limit is advantageous for thematerial from which the mold is made.

It is therefore to be expected that the same rigidity as that of, theformer chill-molds with a wall thickness of 35 mms. can be obtained in aberyllium copper chill mold with a wall thickness of about 8 mms. Such amold would not weaken the magnetic rotary field more than a brass tubewith a wall thickness of 6 to 7 mms.

I claim:

1. A continuous casting mold for casting metal comprising a mold bodyhaving a cooled shape-giving part, electrical means surrounding saidpart for forming an exteriorly applied rotating magnetic field forproducing a rotating electrical field in the metal being cast in themold, said body being composed of a material which has a heatconductivity greater than 0.094 but less than that of copper and whichhas a higher Wiedemann-Fra'ntz law constant than that for copper alone.7

2. A continuous casting mold asin claim '1, said mold having ashape-giving cooled part at least 6 mm. thick.

3. A continuous casting mold as in claim 1, further comprising ashape-giving portion composed of brass.

4. A continuous casting mold as in claim 1, further comprising ashape-giving portion composed of M863 brass. Y

5. A continuous casting mold as in claim 1, further comprising ashape-giving part composed of a material selected from the groupconsisting of W, Cr, Be, and Mo.

6. A continuous casting mold as in claim 1, further comprising ashape-giving part composedv of silicon.

7. A continuous casting mold as in claim 1, further comprising hardenedberyllium copper containing 2 percent beryllium.

8. A continuous casting mold as in claim 7, said shapegiving part havinga wall thickness of about 8 mm.

References Cited in the file of this patent UNITED STATES PATENTS1,920,699 Hurley Aug. 1, 1933 2,245,224 Poland June 10, 1941 2,284,704\Nelblund et al June 2, 1942 2,631,356 Sparks et a1. Mar. 17, 1953FOREIGN PATENTS 375,304 Great Britain June .16, 1932 504,519 GreatBritain Apr. 26, 1939 667,473 Great Britain Mar. 5, 1952 804,368 GermanyApr. 23, 1951 o... Air

